Mixed rare earth metal, divalent transition metal, aluminum oxide

ABSTRACT

A mixed aluminum oxide having the following formula: 
     
         X.sup.1.sub.(x.sbsb.1.sub.-x.sbsb.2.sub.) X.sup.2.sub.(x.sbsb.2.sub.) 
    
      M 1 .sub.(y.sbsb.1 -y .sbsb.2.sub.) M 2 .sub.(y.sbsb.2.sub.) Al.sub.(z.sbsb.1 -z .sbsb.2.sub.) M 3 .sub.(z.sbsb.2.sub.) O 19   
     wherein X 1  and X 2 , which can be the same or different, represent a metal selected from the group consisting of lanthanum, praseodymium, neodymium, samarium and gadolinium, provided that X 1  and X 2  are different when X 1  or X 2  represents lanthanum or gadolinium; M 1  and M 2 , which can be the same or different, each represent a metal selected from the group consisting of magnesium and divalent transition metals; M 3  represents a trivalent transition metal; x 1  represents a number from 0.8 to 1; y 1  represents a number from 0.7 to 1; z 1  represents a number from 10 to 12; x 2  represents a number from 0 to x 1  ; y 2  represents a number from 0 to y 1  ; and z 2  represents a number from 0 to 1; and wherein said mixed aluminum oxide has a single phase crystalline structure of the magnetoplumbite type. A process for producing the mixed oxide is also disclosed, as well as its applications to power lasers emitting in the infrared and to telecommunications by optical fibers.

This is a continuation of application Ser. No. 281,396, filed July 8,1981 now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to mixed aluminium oxides, theirproduction process and their use.

More specifically the present invention relates to mixed aluminiumoxides obtained in the form of monocrystals and more particularly usedin the field of microlasers for integrated optics or telecommunicationsby optical fibres, as well as in the field of power lasers emitting inthe infrared permitting, for example, the processing of materials, theperformance of photochemical reactions and controlled thermonuclearfusion.

At present mixed aluminium oxides are known and in particular mixedaluminates of lanthanum-magnesium, cerium-magnesium, lanthanum-manganeseand cerium-manganese of chemical formula LnMAl₁₁ O₁₉ in which Ln standsfor the lanthanum or cerium and M for the magnesium or manganese. Thesemixed aluminium oxides produced by the Philips company can be doped byluminescent ions such as Ce³⁺, Tb³⁺, Eu²⁺, Mn²⁺ ions.

These doped or undoped mixed oxides obtained only in the form of powdershave important luminescent properties (display device, etc.), but haveno laser effect. Their production processes and their optical properties(luminescence) are described in a number of articles such as, forexample, an article entitled "Cerium and Terbium luminescence inLaMgAl₁₁ O₁₉ ", published in the Journal of Luminescence 6, 1973, pp.425 to 431 and in another article entitled "Red Mn²⁺ luminescence inhexagonal aluminates" published in the Journal of luminescence 20, 1979,pp. 99 to 109.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to mixed aluminium oxides obtained in theform of monocrystals and which in particular have optical propertiespermitting their use as a laser emitter.

The present invention relates to mixed aluminium oxides, wherein theyhave the following formula:

    X.sup.1.sub.(x.sbsb.1.sub.-x.sbsb.2.sub.) X.sup.2.sub.(x.sbsb.2.sub.) M.sup.1.sub.(y.sbsb.1.sub.-y.sbsb.2.sub.) M.sup.2.sub.(y.sbsb.2.sub.) Al.sub.z.sbsb.1.sub.z.sbsb.2.sub.) M.sup.3.sub.(z.sbsb.2.sub.) O.sub.19

in which X¹ and X², which can be the same or different represent a metalin the group of rare earths, provided that X¹ and X² are different whenX¹ or X² stand for cerium, lanthanum or gadolinium and provided thatwhen X² stands for gadolinium, lanthanum, europium, terbium ordysprosium X¹ stands for a metal of the group of rare earths other thancerium; M¹ and M², which can be the same or different stand for a metalchosen from the group including magnesium and divalent transitionmetals; M³ stands for a trivalent transition metal; x₁ stands for anumber between 0.8 and 1; y₁ stands for a number between 0.7 and 1; z₁stands for a number between 10 and 12; x₂ represents a number from 0 tox₁ ; y₂ represents a number from 0 to y₁ ; and z₂ represents a numberfrom 0 to 1; and wherein they have a crystalline structure of themagnetoplumbite type.

It should be noted that these mixed aluminium oxides are obviouslysingle-phase compounds.

These mixed aluminium oxides can have a stoichiometric composition suchthat x₁ is equal to 1, y₁ to 1 and z₁ to 11, but may also have aslightly different composition with respect to the relative proportionof the lananide ions, i.e. ions of X¹ and X², the divalent ions, i.e.ions of M¹ and M² and the trivalent ions, i.e. the ions of M³ andaluminium, provided that the crystalline structure of these oxides is ofthe magnetoplumbite type.

Advantageously these mixed aluminium oxides have a composition, otherthan a stoichiometric composition, such that x₁ is equal to 0.95, y₁ to1, z₁ to 11 or such that x₁ is equal to 1, y₁ to 0.9 and z₁ to 11.

Advantageously X¹ and X² are chosen from the group including lanthanum,praseodymium, neodymium, samarium, europium, gadolinium, divalenttransition metals in the group comprising manganese, iron, nickel andcobalt and the trivalent transition metal is chromium.

Preferably, according to the invention, the mixed aluminium oxide is offormula:

    La.sub.0.9 Nd.sub.0.1 MgAl.sub.11 O.sub.19

This mixed oxide is particularly suitable as a laser emitter, because ithas optical properties comparable to those of the yttrium and aluminiumgarnet of formula Y₃ Al₅ O₁₂ in which 1% of the yttrium ions aresubstituted by neodymium ions and known as YAG and the neodymiumultraphosphate (NdP₅ O₁₄) used at present as the emitter for lasersoperating in the infrared.

These mixed oxides are obtained on the basis of a first process, whereinpowders of oxides of X¹, X², M¹, M², M³ and aluminium in appropriateproportions are intimately mixed and the mixture obtained is melted.

According to another feature of this first process the molten mixture istreated so as to obtain a monocrystal by solidifying the mixture.

These mixed oxides can also be obtained by a second process, wherein thehydroxides of salts of X¹, X², M¹, M², M³ and aluminium, dissolved in anon-aqueous solvent in appropriate proportions are coprecipitated andthe coprecipitate obtained is calcined.

According to another feature of this second process the coprecipitate ismelted after the pulverization thereof and the molten coprecipitate isobtained so as to obtain a monocrystal by solidification of the saidcoprecipitate.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in greater detail hereinafter relative tonon-limitative embodiments and the attached drawings, wherein show:

FIG. 1 diagrammatically a device for obtaining a monocrystal from mixedaluminium oxides according to the invention.

FIG. 2 diagrammatically the fluorescence emission diagram of the mixedoxide Nd₀.1 La₀.9 MgAl₁₁ O₁₉.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In exemplified manner we will describe hereinafter the production of themixed aluminate of neodymium-magnesium of formula NdMgAl₁₁ O₁₉.

For this purpose the aluminium oxide in powder form, supplied byIndustrie des Pierres Scientifiques H. Djevahirdjian, 1870 Monthey,Switzerland, neodymium oxide (Nd₂ O₃) supplied by Rhone-Progil andmagnesium oxide (MgO) of quality R P. (oxide for analysis), supplied byProlabo and also in powder form are intimately mixed, e.g. instoichiometric proportions, i.e. in accordance with the chemicalreaction:

    11Al.sub.2 O.sub.3 +Nd.sub.2 O.sub.3 +2MgO→2NdMgAl.sub.11 O.sub.19

These oxides are in the form of a powder with a grain size distributionbetween 1 and 10μ and a purity level exceeding 99.9%.

Intimate mixing of these powders can take place either in the solidstate in a mortar by pulverization for approximately 30 minutes, or bydispersing the powders in an organic solvent, preferably ether oracetone so as to improve the homogeneity of the mixture. The suspensionis then vigorously stirred for approximately 30 minutes. As the organicsolvent has no chemical function it is then eliminated by evaporation.

In order to obtain e.g. 30.7 g of mixture 6.730 g of neodymium oxide and1.600 g of magnesium oxide are added to 22.400 g of alumina, saidconstituents having the characteristics defined hereinbefore.

The monocrystal of NdMgAl₁₁ O₁₉ can be obtained after melting thisintimate mixture.

For a more complex mixed aluminium oxide containing, for example,besides the constituents referred to hereinbefore lanthanum andchromium, the mixture is formed in the same way by adding in adequateproportions lanthanum oxide (La₂ O₃) and chromic oxide (Cr₂ O₃) inpowder form.

As oxides of lanthanides (La₂ O₃, Nd₂ O₃, . . . ) are very hydroscopicthey must be calcined before mixing them with the other constituents,e.g. at a temperature of approximately 1000° C.

The monocrystal can be obtained either by the zone melting or floatingzone method, or by direct induction of the high frequency in anautomatic crucible, followed by slow cooling from the crucible walls,(method known as the Kyropoulos method) or by drawing from a molten bath(method known as the Czochralski method) or by the Verneuil method. Thelatter method, which is easy to perform makes it possible to obtainlarge monocrystals. It will now be described with reference to theapparatus diagrammatically shown in FIG. 1.

This apparatus comprises a powder distributor 2 having in its lower parta screen 4 on which is placed the previously obtained powder mixture 6.This screen 4 has a mesh width which can vary between 125 and 210μ, soas to permit the passage of the powders. Powder distributor 2 alsocomprises an impact or percussion device, constituted by a hammer 8 anda rod 10 in contact with the bottom of screen 4.

When hammer 8 strikes rod 10, the latter starts to vibrate, thus causinga certain quantity of powder to fall in the direction of a nozzle 12 ofan oxyhydrogen blowpipe. The speed of the hammer blows can vary between10 and 40 blows per minute, which causes 1 to 6 milligrams of powder todrop at each impact.

The oxyhydrogen blowpipe makes it possible to melt the powder mixture 6.The blowpipe is supplied with oxygen by means of a delivery pipe 14located at the top of the distributor so as to drive the powder 6towards the blowpipe nozzle 12. Hydrogen is supplied by delivery pipe16. The oxygen flow rate varies between 4 and 7 liters per minute (l/mm)at a pressure of approximately 10 millibars (mb) and is controlled by asystem of manometers and flowmeters diagrammatically indicated at 18.The hydrogen flow rate varies between 12 and 21 l/mn at a pressure ofapproximately 2 mb and is controlled in the same way by means of asystem 20.

The powder mixture 6 melted by means of the blowpipe is then depositedon an alumina support 22 placed within a clay muffle 24, whose innerfaces are covered with alumina.

Muffle 24 has a window 26 which can be associated with an optical device28 making it possible to check the formation of monocrystal 30 and tomaintain the crystallization front at a constant level. Thecrystallization front is maintained in blowpipe flame 32 by means of anautomatic device 34 enabling support 22 to be lowered. Support 22 canalso be rotated by an automatic device 36.

In order to produce the mixed aluminium oxide monocrystal following thestarting up of the blowpipe the high powder flow rate is fixed so as toobtain on support 22 a fritted cone 38 of mixed oxide which has a heightof approximately 10 mm. The fall of powder mixture 6 is then stopped andblowpipe flame 32 regulated so as to bring about the melting of themixed oxide. A molten drop which will give the crystallization nucleusthen forms at the apex of cone 38. This drop can spread over a diametervarying between 3 and 10 mm. The growth of the monocrystal is initiated.The powder supply is then regulated (number of hammer blows per minute)so as to maintain the crystallization front at a constant level, i.e. inblowpipe flame 32, as well as a processing speed for monocrystal 30 ofapproximately 10 to 20 mm/hour.

At the end of the operation the supply of powder 6 is stopped andmonocrystal 30 is progressively removed from the blowpipe flame 32.After stopping the blowpipe monocrystal 30 is cooled in muffle 24 makingit possible to prevent any sudden temperature variation. A suddentemperature variation could cause cracks in the monocrystal.

Furthermore, to prevent a too rapid cooling of monocrystal 30 the lowerpart 40 of the muffle is closed. When the monocrystal is completelycooled it is merely necessary to separate it from cone 38 by a singlesaw stroke.

In this way it is possible to obtain a sample e.g. of NdMgAl₁₁ O₁₉having a diameter of approximately 15 mm and a length of 50 mm.Processing of such a sample can last 5 to 8 hours and cooling iscontrolled by the thermal inertia of muffle 40.

It should be noted that all the mixed oxides according to the inventionhave a crystalline growth close to that of alumina. It would appear tobe no more difficult to produce them in the form of monocrystals than toproduce alumina monocrystals.

The monocrystal of NdMgAl₁₁ O₁₉ as well as the other mixed aluminiumoxides according to the invention can be obtained by another productionprocess. This process for the oxide of NdMgAl₁₁ O₁₉ consists ofcoprecipitating in amorphous form (i.e. very reactive form) thehydroxides of the various elements Nd, Mg and Al.

Firstly three solutions are formed, which are then mixed in appropriateproportions, e.g. in stoichiometric proportions to obtain the mixedoxide of formula NdMgAl₁₁ O₁₉.

The first solution is constituted by neodymium oxide (Nd₂ O₃) which isdissolved in highly concentrated hydrochloric acid enabling thecorresponding chloride to be obtained. The second solution is constituedby magnesium chloride (MgCl₂) which is dissolved in absolute alcohol.The third solution is constituted by aluminium chloride (AlCl₃) alsodissolved in absolute alcohol.

When the mixture of the three solutions has been formed thecorresponding hydroxides are coprecipitated, preferably by ammonia.Certain hydroxides, such as magnesium hydroxide are soluble in ammoniain aqueous solution, so that coprecipitation is performed in anon-aqueous solvent such as absolute alcohol.

The thus obtained coprecipitate is then calcined at a temperature ofapproximately 500° C. so as to eliminate the ammonium chloride (NH₄ Cl)formed during ammonia addition. The thus calcined coprecipitate is thenpulverized.

As previously it is possible to obtain the monocrystal NdMgAl₁₁ O₁₉ fromthis coprecipitate by the Verneuil method or by any of the other methodsreferred to hereinbefore.

The second process according to the invention has the interest of givinga very homogeneous mixture, but it cannot be used in a general basis forall the mixed oxides according to the invention for various chemicalreasons. However, the first process can be used with all the mixedaluminium oxides according to the invention, provided that certainoperating modifications in connection with the production of themonocrystal are made, as a function of whether the basic constituentsfor producing the monocrystal have a greater or lesser reducing oroxidizing action.

These mixed oxides obtained as hereinbefore have a hexagonal structureintermediate between the magnetoplumbite structure of formula BaAl₁₂O₁₉, the sodium alumina β structure and the silver alumina β structure,which are well known in the art.

The various mixed oxides obtained in the form of monocrystals accordingto the aforementioned processes and in particular those containingneodymium have optical properties permitting their use as a laseremitter.

It is known that the laser effect is dependent on certain parameters andin particular the life of the excited state of the ions (energy levelabove the fundamental level). If the life of the excited state issufficient (several dozen microseconds) population inversion can takeplace (the number of ions in the excited state exceeds the number ofions in the fundamental state).

Research has shown that the presence of neodymium, associated withlanthanum or gadolinium in the mixed oxides according to the inventionmakes it possible to obtain sufficiently long lives to bring about thispopulation inversion, gadolinium and lanthanum serving the samefunction. In mixed oxides the excited state corresponds to the energylevel ⁴ F_(3/2) of the Nd³⁺ ion.

The life of the excited state ⁴ F_(3/2) as a function of the quantity ofNd³⁺ ions per cubic centimeter of monocrystal will now be shown formixed oxides of formula Nd.sub.(1-x) La.sub.(x) MgAl₁₁ O₁₉ with xbetween 0 and 1. The final compound of formula Y₃ Al₅ O₁₂ with 1% ofneodymium, called YAG is given for comparison purposes (see attachedtable).

This table shows that the more lanthanum contained by the mixed compoundthe longer the life of the excited state. Moreover, for a concentrationof Nd³⁺ ions between 0.03 and 0.34×10²¹ /cm³ the life of the excitedstate of a mixed oxide according to the invention is longer than that ofthe excited state for the reference compound used at present as theemitter for lasers.

It should be noted that a mixed oxide which does not contain lenthanum,i.e. of formula LaMgAl₁₁ O₁₉ has no laser effect. The latter is due tothe presence of neodymium in the mixed oxides according to theinvention. The other mixed oxides according to the invention whichcontain praseodymium, samarium or europium as the lanthanide and whichmay or may not be associated with lanthanum or gadolinium haveinteresting luminescent properties.

The optical properties (luminescence or laser effect) are characteristicof the lanthanides referred to hereinbefore, so that the magnesiumpresent in the mixed oxide in question may wholly or partly be replacedby manganese, iron, nickel and cobalt in divalent form, said elementsserving the same function.

It is also known that the laser effect is dependent on the oscillatorstrength of the transitions in the luminescent ions. This oscillatorstrength must be as high as possible. The mixed oxides according to theinvention have oscillator strengths, when light is propagated along theC axis of the crystal, which are close to those e.g. of neodymiumultraphosphate.

The fluorescence intensity increases with the concentration of neodymiumions, but too high a quantity of Nd³⁺ ions aids interactions betweenluminescent ions which are prejudicial to fluorescence. As the presenceof lanthanum makes it possible to increase the fluorescence yield by adilution effect the mixed oxide of formula Nd₀.1 La₀.9 MgAl₁₁ O₁₉ seemsto be a good compromise with regards to the quantity of Nd³⁺ ions.

A good fluorescence yield can be obtained by using basic constituentshaving a high purity (above 99.9%) for the production of the mixedoxides according to the invention having a laser effect. The presence ofimpurities in the monocrystal would lead to fluorescence losses, becauseneodymium ions in the excited state could be deactivated to thefundamental state in a non-radiative process.

Research has shown that mixed oxides containing neodymium have threetypes of emission, shown in FIG. 2, between the excited state ⁴ F_(3/2)having an energy E₁ and the state ⁴ I having four sublevels ofincreasing energy E ⁴ I_(9/2), ⁴ I_(11/2), ⁴ I_(13/2), ⁴ I_(15/2).

The emission between the excited level E₁ and level ⁴ I_(9/2) has awavelength λ₁ close to 0.89μ, the emission between the excited level E₁and level ⁴ I_(11/2) has a wavelength λ₂ close to 1.06μ and emissionbetween the excited level E₁ and level ⁴ I_(13/2) has a wavelength λ₃close to 1.32μ.

Emission at the wavelength 1.06 makes it possible to use these mixedoxides in lasers working in the infrared.

Emissions at wavelengths of 1.06 and 1.32μ should make it possible touse these mixed oxides in the field of telecommunications by opticalfibres. Thus, optical fibres, e.g. of silica, which to a greater orlesser extent absorb the wavelengths to be transmitted, have a lowattenuation at these wavelengths, so that a maximum of information canbe transmitted.

To bring about the population of the excited state ⁴ F_(3/2) of energyE₁ the atoms are excited up to an energy level E₂, such that E₂ exceedsE₁. The population of the energy state E₂ is obtained by lightabsorption. The energy level E₂ is very unstable, so that the Nd³⁺ ionsare spontaneously deenergized to the energy state E₁, state ⁴ F_(3/2).This method of populating the excited state ⁴ F_(3/2) is known asoptical pumping. As the neodymium absorption peaks are very narrow incertain cases the light absorption of these mixed oxides can be aided bysubstituting e.g. 1% of the aluminium ions by transition metal ions andpreferably by chromium. Thus, in the visible range the latter has twomuch wider absorption bands (400 to 500 and 600 to 700 nanometers).

The energy due to light absorption by chromium can be transmitted to theNd³⁺ ions, permitting the population of the energy level E₁.

The physical properties of the mixed oxides according to the invention,like the mechanical and thermal properties are close to the physicalproperties of alumina.

Research on the magnetic properties carried out on the oxide NdMgAl₁₁O₁₀ have shown that this oxide has an important anisotropy of itsmagnetic susceptibility mainly at low temperatures.

Mixed oxides containing neodymium associated with gadolinium orlanthanum have optical properties making it possible to use them as anemitter for lasers working in the infrared range and which can forexample be used in the processing of materials or telecommunications byoptical fibres.

                  TABLE                                                           ______________________________________                                                                    Life of excited                                                  Number of Nd.sup.3+                                                                        state .sup.4 F.sub.3/2 in                         Mixed oxides   ions/cm.sup.3 (×10.sup.21)                                                           microseconds                                      ______________________________________                                        NdMgAl.sub.11 O.sub.19                                                                       3.36          27                                               Nd.sub.0.33 La.sub.0.66 MgAl.sub.11 O.sub.19                                                 1.12          52                                               Nd.sub.0.1 La.sub.0.9 MgAl.sub.11 O.sub.19                                                   0.34         260                                               Nd.sub.0.05 La.sub.0.95 MgAl.sub.11 O.sub.19                                                 0.17         360                                               Nd.sub.0.01 La.sub.0.99 MgAl.sub.11 O.sub.19                                                 0.03         360                                               Y.sub.3 Al.sub.5 O.sub.12 with 1% Nd.sup.3+                                                  0.14         200 to 240                                        ______________________________________                                    

What is claimed is:
 1. A mixed aluminum oxide having the followingformula:

    X.sup.1.sub.(x.sbsb.1.sub.x.sbsb.2.sub.) X.sup.2.sub.(x.sbsb.2.sub.) M.sup.1.sub.(y.sbsb.1.sub.-y.sbsb.2.sub.) M.sup.2.sub.(y.sbsb.2.sub.) Al.sub.(z.sbsb.1.sub.-z.sbsb.2.sub.) M.sup.3.sub.(z.sbsb.2.sub.) O.sub.19

wherein X¹ represents a metal selected from the group consisting oflanthanum, praseodymium, neodymium, samarium and gadolinium, and X²represents a metal selected from the group consisting of praseodymium,neodymium, samarium, X₁ and X₂ being the same or different; M¹ and M²,which can be the same or different, represent a metal selected from thegroup consisting of magnesium and divalent transition metals; M³represents a trivalent transition metal; x₁ represents a number from 0.8to 1; y₁ represents a number from 0.7 to 1; z₁ represents a number from10 to 12; x₂ represents a number from 0 to x₁, x₂ being different from 0when X₁ represents lanthanum or gadolinium; y₂ represents a number from0 to y₁ ; and z₂ represents a number from 0 to 1; and wherein said mixedaluminum oxide has a single phase crystalline structure of themagnetoplumbite type.
 2. A mixed oxide according to claim 1, wherein x₁is equal to 1, y₁ to 1 and z₁ to
 11. 3. A mixed oxide according to claim1, wherein x₁ is equal to 0.95, y₁ to 1 and z₁ to
 11. 4. A mixed oxideaccording to claim 1, wherein x₁ is equal to 1, y₁ to 0.9 and z₁ to 11.5. A mixed oxide according to claim 1, wherein the transition metals M¹and M² are selected from the group consisting of magnanese, iron, nickeland cobalt.
 6. A mixed oxide according to claim 1, wherein M³ ischromium.
 7. A mixed oxide according to claim 1, wherein X¹ representslanthanum or gadolinium, X² neodynium and M¹ magnesium.
 8. A mixed oxideaccording to claim 7, wherein x₂ is equal to 0.1.
 9. A mixed oxide ofthe formula:

    La.sub.0.9 Nd.sub.0.1 MgAl.sub.11 O.sub.19

having a single phase crystalline structure of the magnetoplumbite type.10. A mixed aluminum oxide of the formula:

    Nd.sub.(1-x) La.sub.(x) MgAl.sub.11 O.sub.19

wherein x stands for a number from 0 to 0.99 and having a single phasecrystalline structure of the magnetoplumbite type.
 11. A mixed aluminumoxide having the following formula:

    X.sup.1.sub.(x.sbsb.1.sub.-x.sbsb.2.sub.) Nd.sub.(x.sbsb.2.sub.) M.sup.1.sub.(y.sbsb.1.sub.-y.sbsb.2.sub.) M.sup.2.sub.(y.sbsb.2.sub.) Al.sub.(z.sbsb.1.sub.-z.sbsb.2.sub.) M.sup.3.sub.(z.sbsb.2.sub.) O.sub.19

wherein X¹ represents lanthanum or gadolinium; M¹ and M², which can bethe same or different, each represent a metal selected from the groupconsisting of magnesium and divalent transition metals; M³ represents atrivalent transition metal; x₁ represents a number from 0.8 to 1; y₁represents a number from 0.7 to 1; z₁ represents a number from 10 to 12;x₂ represents a number within the range of 0<x₂ ≦x₁ ; y₂ represents anumber from 0 to y₁ ; and z₂ represents a number from 0 to 1; andwherein said mixed aluminum oxide has a single phase crystallinestructure of the magnetoplumbite type.
 12. The mixed oxide according toclaim 11, wherein x₁ is equal to 1, y₁ is equal to 1 and z₁ is equal to11.
 13. The mixed oxide according to claim 11, wherein x₁ is equal to0.95, y₁ is equal to 1 and z₁ is equal to
 11. 14. The mixed oxideaccording to claim 11, wherein x₁ is equal to 1, y₁ is equal to 0.9 andz₁ is equal to
 11. 15. The mixed oxide according to claim 11, whereinthe transition metals M¹ and M² are selected from the group consistingof manganese, iron, nickel and cobalt.
 16. The mixed oxide according toclaim 11, wherein M³ is chromium.
 17. The mixed oxide according to claim11, wherein M¹ is magnesium.
 18. The mixed oxide according to claim 17,wherein x₂ is equal to 0.1.